docs: def _is_dirty(self, xblock):
          """
          Return whether this field should be saved when xblock.save() is called
          """
          # pylint: disable=protected-access
          if self not in xblock._dirty_fields:
              return False
  
          baseline = xblock._dirty_fields[self]
          return baseline is EXPLICITLY_SET or xblock._field_data_cache[self.name] != baseline